The Syrian military and the country's main rebel force have agreed to halt operations from today for the Muslim holiday weekend.

United Nations-Arab League peace envoy Lakhdar Brahimi proposed the truce and had earlier said president Bashar al-Assad was ready to observe it.

"On the occasion of Eid al-Adha, military operations will cease on Syrian territory as of Friday morning, until Monday the 29th," the Syrian army confirmed overnight in a statement read on state television.

But it also said the armed forces reserved the right to respond to actions by the rebel forces, which Mr Assad's regime describes as terrorists.

The statement said the army would react if "armed terrorist groups continue to fire on civilians and government troops, attack public and private property and use car bombs and improvised explosive devices".

It also warned of a response if rebels "strengthen their current positions or continue to receive reinforcements and ammunition" and to any fighters crossing from neighbouring countries.

The statement says the ceasefire will begin at 2:00pm (AEDT).

Syria's main rebel force, the Free Syrian Army (FSA), said it had also agreed to temporarily halt military operations from Friday morning but would similarly respond to attacks.

"We will respect the ceasefire from tomorrow morning if the Syrian army does the same," FSA general Mustafa al-Sheikh said.

"But if they fire a single shot, we will respond with 100. So we reserve the right to respond."

However, Mr Sheikh said he could not speak on behalf of all rebel groups.

"There is not a unified command for all the factions," he said.

"We speak on behalf of a big enough number of fighters, but there are other armed factions who follow other commands."

But other rebels groups have refused to accept the proposal, with the radical Islamic Al-Nusra Front saying it will not lay down its weapons and denouncing the truce as a "trick".

The Eid holiday starts with morning prayers at dawn on Friday, expected around 5:30am local time in Syria.

If followed through, a ceasefire would mark the first real halt in the 19-month conflict that rights groups say has killed more than 35,000 people.

Fighting continued to rage on the eve of the holiday, with rebel forces clashing with regime troops as they moved into new areas of the second city of Aleppo and battles also continuing elsewhere.

Aid on standby

The UN's refugee agency says that if the ceasefire holds, it is ready to send emergency aid to thousands of Syrian families in previously inaccessible areas.